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Public health is concerned with threats to the overall health of a community based on population health analysis. Many organizations define health and how to promote health differently. The World Health Organization, the United Nations body that sets standards and provides global surveillance of diseases and epidemics, defines health as: "A state of complete physical, mental and social well-being and not merely the absence of disease or infirmity."

The focus of a public health intervention is to prevent rather than treat a disease through surveillance of cases and the promotion of healthy behaviors. In addition to these activities, in many cases treating a disease can be vital to preventing it in others, such as during an outbreak of an infectious disease. Vaccination programs and distribution of condoms are examples of public health measures.

There is a vast discrepancy between access to healthcare and public health intiatives between developed nations and developing nations. In the developing world, many public health infrastructures are still forming. There may not be enough trained health workers or monetary resources to provide even a basic level of medical care and disease prevention. As a result, a large majority of disease and mortality in the developing world results from and contributes to extreme poverty. In many countries in Africa, governments spend less than USD$10 per person on healthcare, while, in the United States, the federal government spent approximately USD$4,500 per capita in 2000.

Many diseases are preventable through simple, non-medical methods. Public health plays a very important role in prevention efforts in the developing world, either through the local health system or through international non-governmental organizations.

The major postgraduate degree related to this field is the Master in Public Health (M.P.H.) or Master of Health Science, while the United States medical residency specialty is General Preventive Medicine and Public Health.

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In some ways, public health is a modern concept, although it has roots in antiquity. From the early beginnings of human civilization, it was recognized that polluted water and lack of proper waste disposal may spread vector-borne diseases. Early religions attempted to regulate behavior that specifically related to health, from types of food eaten, to the extent which certain behaviors could be indulged, such as drinking alcohol or sexual relations. The establishment of governments placed responsibility on leaders to develop public health policies and programs to gain some understanding of the causes of disease to ensure stability, prosperity, and maintain order.

By Roman times, it was well understood that proper diversion of human waste was a necessary tenet of public health in urban areas. The Chinese developed the practice of variolation following a smallpox epidemic around 1000 BC An individual without the disease could gain some measure of immunity against it by inhaling the dried crusts that formed around lesions of infected individuals. Also, children were protected by innoculating a scratch on their forearms with the pus from a lesion. This practice was not documented in the West until the early-1700s, and was used on a very limited basis. The practice of vaccination did not become prevalent until the 1820s, following the work of Edward Jenner to treat smallpox.

During the 14th century Black Death in Europe, it was believed that removing bodies of the dead would further prevent the spread of the bacterial infection. This did little to stem the plague, however, which was actually spread by rodent-borne fleas. Burning areas of cities resulted in much greater benefit, since it removed the rodent infestations. The development of quarantine in the medieval period helped mitigate the effects of other infectious diseases. However, according to Michel Foucault, the plague model of governmentality was to be opposed to the later cholera model. Cholera, which second pandemic devastated Europe between 1829 and 1851, was first fought by the use of what Foucault called "social medecine", which focused on flux, circulation of air, location of cemeteries, etc. All those concerns were thus mixed with urbanistic concerns of the management of populations, which Foucault designed by the concept of "biopower".

The science of epidemiology was founded by John Snow's identification of a polluted public water well as the cause of an 1854 cholera outbreak in London. John believed in the germ theory of disease as opposed to the prevailing miasma theory. Although miasma theory taught correctly that disease is a result of poor sanitation, it was based only upon the prevailing theory of spontaneous generation. Microorganisms, which are now known to cause many of the most common infectious diseases, were first observed around 1680 by Anton van Leeuwenhoek. But the modern era of public health did not begin until the 1880s, when the culmination of Robert Koch's germ theory and Louis Pasteur's production of artificial vaccines revolutionized the study of infectious disease.

As the rate of infectious diseases in the developed world decreased through the 20th century, public health began to put more focus on chronic diseases such as cancer and heart disease. Meanwhile, the developing world remained plagued by largly preventable infectious diseases, exacerbated by malnutrition and poverty.

Since the 1980s, the growing field of population health has broadened the focus of public health from individual behaviors and risk factors to population-level issues such as inequality, poverty, and education. Richard Wilkinson has been a major contributor to this area of study.

Modern public health is often concerned with the addressing determinants of health across a population, rather than advocating for individual behaviour change. There is a recognition that our health is affected by many factors including where we live, genetics, our income, our educational status and our social relationships - these are known as "social determinants of health." A social gradient in health runs through society, with those that are poorest generally suffering the worst health. However even those in the middle classes will generally have worse health outcomes than those of a higher social stratum (WHO, 2003). The new public health seeks to address these health inequalities by advocating for population-based policies that improve the health of the whole population in an equitable fashion.

Today, most governments recognize the importance of public health programs in reducing the incidence of disease, disability, and the effects of aging, although public health generally receives significantly less government funding compared with medicine. In recent years, public health programs providing vaccinations have made incredible strides in promoting health, including the eradication of smallpox, a disease that plagued humanity for thousands of years.

One of the most important public health issues facing the world currently is HIV/AIDS. Tuberculosis, which claimed the lives of authors Franz Kafka and Charlotte Bronte, and composer Franz Schubert, among others, is also reemerging as a major concern due to the rise of HIV/AIDS-related infections and the development of strains resistant to standard antibiotics.

A controversial aspect of public health is that related to the control of smoking. Many nations have implemented major initiatives to cut smoking, such as increased taxation and bans on smoking in some or all public places. Proponents argue that smoking is one of the major killers in all developed countries, and that they have a duty to reduce the death rate, both through limiting passive smoking and by providing fewer opportunities for smokers to smoke. Opponents say that this undermines individual freedom and personal responsibility, (often using the phrase nanny state in the UK) and worrying that the state may take power to remove more and more choice in the name of better population health overall.

The application of health economics to the realm of public health has been rising in importance since the 1980s. Health economics studies can show, for example, where limited public resources might best be spent to save lives or cause the greatest increase in quality of life.